When data or control signals are issued to a magnetic disk drive from a host unit which controls output/input of data, the magnetic disk drive moves an actuator carrying magnetic heads toward one of plural tracks which are formed in concentric circles around a rotating axis of a magnetic disk medium (seek operation), and positions the actuator on a target track (following operation). Next the disk drive writes data on an appointed area (sector) in the target track or reads out data from the appointed area. Herein, each track has plural fan formed recording areas and each of these areas is typically called a sector.
On the other hand, when data or control signals have not been issued to a magnetic disk drive from a host unit, namely, when there is no access operation, and the magnetic disk drive does not work as an external storage device, an idle seek operation is executed in consideration of data reliability. In this regard, an idle seek operation is an operation wherein, when no access is ordered by the host unit, an actuator is moved in a radial direction on a rotating magnetic disk medium.
When a head follows a constant track area on a magnetic disk medium continuously for a long time, it is possible that the magnetic head will crash or come into a physical contact with the track area, whereby data written on the track area may be lost. The main purpose of the idle seek operation is to prevent data loss caused by this problem, although the idle seek operation provides an additional effect, such as to remove dust stuck to magnetic heads.
Certain features concerning the idle seek operation have been disclosed. For example, Japanese patent unexamined publication 07-182807 indicates that an idle seek operation is executed under servo control utilizing standard positioning information on the magnetic disk medium for the purpose of reducing the influence of a mechanical displacement caused by a lapse of time. In this method, it is necessary always that an electronic circuit (read-write circuit) is powered up to read out the servo information from the magnetic disk medium, and sense current is provided to an MR head, which is a magnetic head for reading out data.
In the conventional idle seek operation, as described above, which is executed under servo control utilizing the positioning information on the magnetic disk medium, it is necessary that an electronic circuit for reading the information is powered up at all times to use the positioning information recorded on the magnetic disk medium. Thus, it is difficult to the reduce power consumption of the electronic circuit. Further, to drive an MR head having a magneto-resistive effect element, a sense current has to be supplied to the magneto-resistive effect element; therefore, this not only prevents the MR head from having a long life, but increases power consumption as well.
An increase of the power consumption causes an increase of the battery capacity in a portable system, such as a notebook personal computer, etc., which carries a magnetic disk drive, and a power saving of the magnetic disk drive is required by the portable host system.
An idle seek method, in which the positioning information on the magnetic disk medium is not utilized, is disclosed in Japanese patent unexamined publication 07-262539. In this publication, an idle seek method is disclosed in which an actuator is moved under velocity control utilizing voltage generated by the back electromotive force (hereinafter referred to as back electromotive force) in a voice coil motor (VCM) without using the servo information from the magnetic disk medium, namely without providing the sense current to an MR head and without reading out the positioning information on the magnetic disk medium.
However, in this conventional technology, the actuator is moved so that the back electromotive force is balanced with a voltage-supplied to the VCM; namely, this operation is based on an open loop control by a kind of analog circuit in the form of hardware. Actually, we, the inventors of the present invention, found that, because the back electro-motive force was affected and varied by a variation of VCM temperature, accurate control of the idle seek operation could not be achieved, and so the idle seek operation according to this conventional technology did not have sufficient reliability.
At the moment of start of an idle seek operation, the temperature of a coil in the VCM is relatively high, and, as the coil is cooled gradually, the back electro-motive force is affected and varied, so that accurate velocity control can not be attained. Accordingly, the back electromotive force has to be calibrated adequately by the control information read out from the magnetic disk medium.